Defluorination of phosphate rock



June 1951 c. A. HOLLINGSWORTH 2,5 4

DEFLUORINATION OF PHOSPHATE ROCK 2 Sheets-Sheet 1 Filed Nov. 28, 1950 INVENTOR Cu/vM/v A, Houuvaswzm TH ATTORNEYS June 1951 c. A. HOLLINGSWORTH 6,54

DEFLUORINATION 0F PHQSPHATE ROCK 2 Sheets-Sheet 2 Filed Nov. 28, 1950 m 5 R Y m m m M in -W M m w m 4 m N w 0 T W a J J A? w m M Patented June 12, 1951 DEFLUORINATION 0F PHOSPHATE ROCK Clinton A. Hollingsworth, Lakeland, Fla., assignor to Coronet Phosphate Company, New York, N. Y., a corporation of New York Application November 28, 1950, Serial No. 197,889

(Cl. '7l4'7) 2 Claims.

This invention relates to the defluorination of phosphate rock and similar natural phosphate materials, and has for its object the provision of certain improvements in the defiuorinating of such materials by calcination. I In my copending application for Letters Patent of the United States, Ser. No. 69,985, filed January 8, 1949, a method of defiuorinating phosphate rock by calcination is disclosed in which the calcining charge, initially incapable of being calcined at a temperature of 2500 F.

Without substantial fusion, progressively moves through a kiln at a progressively increasing temperature, and the fusion tempertaure of the charge near the firing or hot end of the kiln is raised to above 2500 F, by blowing finely divided lime into that end of the kiln. I have now discovered that by blowing the finely divided lime into the kiln at a point between the fiame at the firing end of the kiln and the hot surface of the charge, the lime serves as a screen and protects the charge from the intensely hot flame, thereby further insuring passage of the charge through the kiln without substantial fusion.

Based on that discovery, the present invention involves, in a method of calcining phosphate rock in a kiln having a fuel burner at the hot end thereof and through which the calcining charge passes at progressively increasing temperature, blowing finely divided lime into the hot end of the kiln at a point between the fuel burner and the charge. The finely divided lime is preferably soblown into the hot end of the kiln that it fans out into a screen-like layer interposed between the hot products of combustion from the burner and the hot charge, and thereby the charge is protected from the intense heat of the combusting fuel proximate the fuel burner.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a longitudinal sectional elevation of a rotary kiln adapted for thepractice of the invention,

Fig. 2 is a sectional elevational on the section line 2--2 of Fig. 1,

Fig. 3 is a longitudinal sectional elevation (on section line 3--3 of Fig. 4) of a tunnel type of travelling hearth kiln adapted for the practice of the invention, and

Figs. 4, 5, and 6 are sectional elevations on section lines 4-4 of Fig. 3, 5-5 of Fig. 6, and 6-6 of Fig. 5, respectively.

The rotary kiln illustrated in Figs. 1 and 2 of the drawings comprises a cylindrical steel into the cold or feed end of the kiln through a chute it having a feed regulating valve or damper l5. One or more fuel burners it are suitably mounted at the hot or discharge end of the kiln. The fuel burner i6 is generally positioned in about the central longitudinal axis of the kiln. The products of combustion from the fuel burner pass through the kiln to a stack ll having a damper l8. As a consequence of the inclination and rotation of the kiln, the calcining charge progressively passes through the kiln from the cold or feed end to the hot or discharge end and in its passage through the kiln encounters a progressively increasing calcining temperature. The calcined and defluorinated product is discharged into a chute 20. Hoods l9 and 20 are usually provided at the cold and hot ends, respectively, of the kiln.

In accordance with the invention, finely divided lime, preferably in the form of finely ground limestone, is blown into the hot end of the kiln through one or more pipes 2 I. The pipe 2| is positioned between the fuel burner 16 and the bed 22 of the progressively advancing charge, and the blown-in lime forms a screen-like layer 23 of finely divided lime interposed between the hot fiame 24 of the burner and the surface of the hot charge bed 22 near the not end of the kiln. This interposed screen of finely divided lime shields and protects the charge from the very intense heat of the products of combustion proxi mate the burner where the fuel and air discharged therefrom combust. f '1 The tunnel-type kiln or furnace illustrated in Figs. 3, 4, 5 and 6 comprises a travelling hearth made up of refractory brick 25 secured to an endless chain conveyor 26 carried by sprockets 27 and 28, one of which is driven by any suitable source of power (not shown). The hearth travels from left to right in Fig. 3 through a kiln structure or tunnel 29 of refractory material. The calcining charge of phosphate material is fed to the travelling hearth at the cold end of the kiln structure 29 through a chute 30. One or more fuel burners l6 are suitably mounted at the hot end of the kiln 29. The burner- I6 is generally positioned about midway between the hearth and the roof of the kiln. The hot products of combustion from the burner pass through the kiln to a stack l1 having a damper 18'. The calcining charge encounters a progressively increasing temperature as it is carried by the moving hearth from the cold end to the hot end of the kiln. The calcined and defluorinated product is discharged from the hearth into a hopper 3| as the hearth passes around the sprocket wheel 28. The cold and hot ends of the kiln are usually closed by plates or hoods 32 and 33, respectively.

The chain conveyor is made up of alternately overlapping links M and 35 pivoted on axles 35. A plate 37 is suitably secured to each link, and a refractory brick 25 is attached to each plate by a bolt 38. A roller 39 is loosely mounted on each axle S6, and within the kiln the rollers 39 are supported and ride on water-cooled tubular rails 49.

In accordance with the present invention, finely divided lime is blown into the firing and hot end of the kiln 22 through one or more pipes 2|. The pipe 2| is positioned between the fuel burner l6 and the bed 22 of the progressively advancing charge, and the blown-in lime forms a screen-like layer 23 of finely divided lime interposed between the hot flame 24 of the burner and the surface of the hot charge bed 22 near the hot end of the kiln.

-When lime is blown into the firing end of the kiln, the major portion mixes with the charge in the hottest zone of the kiln by settling into contact with and b being readily retained by the hot and slightly tacky charge. The lighter particles of lime remain entrained in the gases passing through the kiln. Thus, there is progressively less settling of lime and mixing thereof with the charge from the hot end of the kiln toward the feed end thereof. The inclusion of lime in the charge generally raises the temperature at which fluorine is otherwise readily removed at temperatures below 2590" F. By blowing lime into the hot end of the kiln, fluorine is substantially eliminated before the charge progresses to a position in the kiln where such an amount of lime becomes mixed with the charge as to substantially impede fluorine removal at temperatures below 2500 F. The rate of movement of the charge through the kiln in such that lime is available near the hot end of the kiln for mixing with the charge in substantial quantities by the time the temperature of the progressively-moving charge approaches that at which fusion thereof, in the absence of lime, would take place, and the lime so mixed with the charge raises the fusion temperature of the charge so that it passes through the hot zone at a temperature of 2609 to 2800 F. without substantial fusion. Without substantial fusion means in the absence of such fusion or sintering as to cause the charge to become sticky. in whole or in part, or tend to make it cling or stick to the walls of the kiln, or, in a rotary kiln, t ball-up and to fail to flow freely and easily through the kiln. The required amount of lime will vary with different phosphate rocks and more particularly with the silica content of such rocks. (calculated as CaO) blown into the kiln will vary from 0.05 to 0.3 the amount by weight of the calcining charge.

When the lime is blown into the kiln at a point between the fuel burner (or burners) and the Generally speaking the amount of lime charge, the finely divided lime performs the additional function of serving as a screen between the flame from the fuel burner and the adjacent charge. The flame resulting from the combustion of the fuel is intensely hot, and the heat radiating from the flame tends to overheat and fuse the surface of the charge directly beneath the name. The interposed screen of finely divided lime protects the charge from the intense heat of the flame, and minimizes local surface fusion or sintering as the charge progressively moves through that part of the hot zone of the kiln where the heating effect of the flame is most intense. Discharged from one or more pipes at the hot end of the kiln, between the charge and the fuel burner, the finely divided lime initially spreads out into a comparatively dense layer between the flame from the burner and the charge. The gaseous turbulence within the kiln and particularly proximate the hot end, caused mostly by the combusting fuel and the kiln draft, promptly sweeps the initial screenlike layer of finely divided lime throughout most of the cross-sectional area of the kiln, but not before the blown-in lime, interposed between the flame and the charge, has served its purpose of protecting the surface of the charge from the intense heat of the flame. Then, as the initial comparatively dense screen-like layer of finely divided lime is dispersed throughout the kiln length, the lime atmosphere within the kiln, and particularly near the firing end, acts to raise the fusion temperature of the charge as it passes through the hot zone of the kiln, as hereinbefore described.

The protecting screen of finely divided lime interposed between the flame and the charge may be used with advantage in any method of defluorinating phosphate rock by calcination in which the position of the flame with respect to the adj acent charge is such that the charge tends to become overheated and to fuse or sinter due to its close proximity to the intensely hot flame. The invention is of particular advantage in those methods of defiuorination by calcination in which the fusion temperature of the charge is raised as it passes through the hot zone of the kiln by the inclusion therein of lime derived from the finely divided lime blown into the kiln at its firing end. Thus, the invention is of special advantage in defluorinating by calcination such mixtures of phosphate rock and phosphoric acid as are described in my aforementioned patent application Ser. No. 69,985 and which are incapable of being calcined at a temperature of 2509" F. without substantial fusion.

Where the appended claims call for blowing lime into the hot end of the kiln, the term lime is intended to include limestone which, as hereinbefore mentioned, decomposes at the temperature prevailing in the hot end of the kiln to yield lime.

I claim:

1. In the method of defluorinating phosphate rock by calcination in a kiln having a fuel burner at the hot end thereof and through which the calcining charge progressively passes from the other end of the kiln to said hot end at progressively increasing temperatures and in which the fusion temperature of the charge is raised near the hot end of the kiln by blowing finely divided lime into the hot end of the kiln, the improvement which comprises blowing the finely divided lime into the kiln at a. point between the fuel burner and the charge.

into the hot end of the kiln, the improvement which comprises interposing near the hot end of the kiln a screen of finely divided lime between the hjojt surface of the charge and the hot products of combustion of the fuel burner by blowing finely divided lime into the kiln at a point between the fuel burner and the charge. i ;i

v CLINTON A. HOLLINGSWORTH.

No references cited. 

1. IN THE METHOD OF DEFLUORINATING PHOSPHATE ROCK BY CALCINATION IN A KILN HAVING A FUEL BURNER AT THE HOT END THEREOF AND THROUGH WHICH THE CALCINING CHARGE PROGRESSIVELY PASSES FROM THE OTHER END OF THE KILN TO SAID HOT END AT PROGRESSIVELY INCREASING TEMPERATURES AND IN WHICH THE FUSION TEMPERATURE OF THE CHARGE IS RAISED NEAR THE HOT END OF THE KILN BY BLOWING FINELY DIVIDED LIME INTO THE HOT END OF THE KILN, THE IMPROVEMENT WHICH COMPRISES BLOWING THE FINELY DIVIDED LIME INTO THE KILN AT A POINT BETWEEN THE FUEL BURNER AND THE CHARGE. 